Fuel system



Get. 22, 1946. R. R. CURTIS FUEL SYSTEM 2 Sheets-Sheet 1 Filed June 4, 1943 $2 V *5 171 527 Czz-v .Twsei/ifia'af as.

R. R. CURTIS FUEL SYSTEM Fil ed June 4, 1945 2 Sheets-Sheet 2 Patented Oct. 22, 1946 FUEL SYSTEM Russell R. Curtis, Dayton, Ohio, assignor to Curtis Pump Company, Dayton, Ohio, a corporation of Ohio Application June 4, 1943, Serial No. 489,651

14 Claims. 1

This invention relates to automatically controlled pumps capable of maintaining predetermined pressure differentials.

Specifically the invention relates to a fuel system especially adapted for aircraft usage to maintain proper feed of fuel to an engine irrespective of the altitude in which the engine is operating.

The invention will hereinafter be specifically described as embodied in an aircraft fuel system, but it should be understood that the features of this invention are useful in many other systems.

In accordance with this invention, the fuel tanks of an airplane are vented to the ambient air, and each carries an individual electric motorcentrifugal booster pump unit capable of separating or rejecting gases and vapors from the liquid fueland pressuring fully liquid fuel into an emergency electric motor-driven booster pump unit. A selector valve is preferably provided between the tank mounted booster pump units and the emergency booster pump unit so that fuel can be selectively fed from one or more tanks into the emergency booster pump unit. The emergency booster pump unit, like the tank pumps, is of thecentrifllgal type having the inlet and outlet thereof in constant communication even when the unit is idle. Normally this emergency unit is idle and the tank units pressure the fuel through the emergency unit to the inlet of a positive displacement main fuel pump driven by the aircraft en gine. The main fuel pump has a by-pass permitting fuel flow therethrough when it is idle and a relief valve mechanism for maintaining a desired pump pressure differential. When the main fuel pump is damaged or insulficient to supply fuel under the desired pressure to the engine, the emergnecy unit is used to supply fuel to the engine at the proper pressure.

The electric motor driving the emergency booster pump is automatically controlled to operate the pump for maintaining a constant fuel pressure differential based on the pressure existing in the engine intake manifold at the point where fuel and air to the engine are admixed.

The tank-mounted booster pumps are usually suflicient to maintain proper engine performance even at high altitudes and, as explained above, the emergency booster pump need not normally be used. However, for emergency high pressure duty, for example in case of failure of the main engine-driven fuel pump, the emergency booster pump is placed into operation to add to the pressure produced by the tank mounted booster pumps, for supplying fuel to the engine through the by-pass in the main fuel pump. Since the tank pump, emergency pump, and main pump are all arranged in series or tandem, and all have actual or inherent by-passes, any one of the three pumps will supply fuel to the engine in amounts sufficient to maintain flight when, for example, the other two pumps are damaged or otherwise inoperative.

The tank-mounted booster pump will pressure the fuel from the tank sufficiently to prevent liberation of gases and vapors from the fuel and, at the same time, will beat out bubbles of gas and vapor from the fuel about to enter the pump so that no vapor lock can occur in the fuel conduit.

The emergency booster pump preferably has a direct current electric motor with a shunt field. A resistance circuit is automatically placed in series relation with the shunt field to weaken the field and speed up the motor whenever the discharge pressure of the emergency pump falls below a predetermined differential pressure. The automatic control of the motor speed is brought about through the use of a motor-driven governor having a first contact point which is radially movable under the influence of centrifugal force cooperating with a second radially movable contact point controlled by a flexible diaphragm. One side of the diaphragm is exposed to pump discharge pressure while the other side of the diaphagm is exposed to engine intake manifold pressure. When the contact points are open, the resistance circuit is placed in series with the shunt field of the electric motor. When the contact points are closed, the resistance circuit is by-passed, the motor field strength increases, and the motor speed reduced.

i It is, then, an object of this invention to provide a fuel system especially useful in aircraft including a tandem arrangement of booster pumps in advance of a main fuel pump for selective operation to insure high altitude performance of the aircraft.

A still further object of the invention is to provide an aircraft fuel system with a tandem arrangement of booster pumps capable of preventing vapor lock in the system while maintaining a constant predetermined fuel pressure differential based on the aircraft engine intake manifold pressure.

A still further object of the invention is to provide an aircraft fuel system with an emergency booster pump capable of maintaining desired fuel pressures even when the main fuel pump of the aircraft engine is inoperative.

Another object of the invention is to provide an electric motor-driven pump wherein the speed aeoaeei 3 of the electric motor is automatically controlled to maintain a desired pump discharge pressure.

A still further object of the invention is to provide a tandem arrangement of electric motor-operated centrifugal type booster pumps with an automatic control for the speed of at least one electric motor.

A still further object of the invention is to provide a booster pump-main fuel pump type of aircraft fuel system with an emergency booster pump which is automatically driven at a speed for maintaining a predetermined fuel pressure differential.

A still further object of the invention is to provide a fuel system for aircraft engines which includes a tandem arrangement of centrifugal type booster pumps wherein at least one of the pumps is driven at a speed determined by the discharge pressure of the pump and the pressure existing in the engine intake manifold at the point where fuel and air are admixed so as to maintain a constant pressure difierential between v the fuel pressure and the engine intake manifold-pressure at this point.

Other and further objects of the invention will be apparent to thoseskilled in the art from the following detailed description of the annexed sheets-of drawings which, by way of preferred eX- ample only, illustrate one embodiment of the inven-tion.

On the drawings:

1 Figure 1 is a fragmentary and broken somewhat diagrammatic elevational view of an aircraft fuel system according to this invention.

1 Figure 2 is a side elevational view, with parts broken away and shown in vertical cross section, of oneof the tank-mounted booster pumps in the system of Figure 1.

- vliigure 3 is .a wiring diagram for the emergency booster. pump'unit' shown in Figure l.

.Figur'e 4 is an enlarged side elevationalview, with parts in vertical cross section, of theemergencybooster pump unit.

- Figure. Eris a view taken along the line .V-V of.

Figure 4.

Figure 6 is a view taken along the line of Figure l, the brush elements being omitted.

I As shown on the drawings:

In Figure 1 the reference numerals 19, Ml des-. ig-n-at e fuel tanksnof an airplane (not shown). These'tanks it, it are vented to the ambient air through vent openings ii; :Min the topwalls thereof. Each. tank 10 has an. electric .motordriven centrifugal booster. pumplzunit -l 2,.mounted on the bottom wall thereof.

As best shown in Figure 2, each booster pump unit 12 includes'a casing .13 housing anielectric mot or (not shown) which is energized through electric wires such as W (Figure 1'). The electric motor has a drive shaft is projecting through a pump casing 55 from the motor casing 13. The pump casing it: has an outturned flange 15a which is directly bolted on the bottom wall of a tank if! as shown in Figure 1.

J The pump casing l5 defines a pump volute chamber 1,6 with a central inlet l1 defined by a throat ring I8 projecting into the tank to freely communicate with the lower portion of the tank.

Fuel can flow by gravity through the inlet l1 into the volute chamber I6 and thence throughan outlet t9 communicating with theperiphery of the volute chamber into apipe line connected; to. an inlet of a selector valve 21.: The narrangement is such that fuel ca'n flow' from the 4 tank I8 into the selector valve 2| even though the pump unit i2 is not being operated.

An impeller 22 (Figure 2) is mounted on the drive shaft l4 and has upstanding pumping vanes 23 thereon underlying the throat ring I8 to centrifugally propel fuel from the inlet I! through the volute chamber l6 and into the pipe line 29. In addition, a propeller 2 4 is mounted on the drive shaft :4 in advance of the pumping vanes 23. This propeller is disposed in the tank and .acts on fuel about to enter the pump unit [2 before the fuel reaches the pumping vanes 23 and is and laterally ejects these bubbles away from the pumping vanes 23 so that, when the pump is operating, only fully liquid fuel is pressured into the fuel line 2'9. The rejected bubbles of gas and vapor will rise to the surface of the fuel in the tank H.) where they will burst to discharge the gases and vapors out of the tank through the vent II.

The selector valve 21 has an outlet communicating with a fuel pipe 25. This fuel pipe 25 conducts fuel from the selector valve 2| to an emergency. duty electric motor-centrifugal booster pump unit 25.

Asshown in Figure 4, the unit 26 includes a generally cylindrical open-ended casing 21 with a cap 28 closing one open end thereof, a cup member 29 seated inthe open end closed by the cap 28 and having a partition wall 29a providing a diaphragm chamber 30 in this end of the casing. A second partition wall 3| is provided intermediate the ends of the casing 21 in spaced relation from the partition wall 29a to provide a governor chamber 32 in the casing. A motor compartment 33 is provided by the casing 21 and houses an electric motor field or stator 34 and an electric motor armature or rotor 35. The armature or rotor 35 is rotatably mounted on a shaft 36 carried by casing-supported bearings such as 31. The casing 27 also carries brushes such as 38 for engaging the commutator 39 of the rotor or armature 35. An impeller 40 is mounted on the shaft 35 and projects beyond the other open end munication irrespective of whether or not the impeller so is being driven. vWhen the impeller Allis being driven, the pumping vanes 40a thereon will centri-fugally pressure fuel from the inlet 43 through the volute 42 intothe outlet 44.

As shown in Figure l, the pipe from the outlet 44 of the unit 26 communicates with the inlet of a main fuel pump 45 mounted on and driven by the aircraft engine 57. The fuel pump 46 which is diagrammatically shown is of the 1505i; tive displacement type including a vaned rotor 46a between the inlet 46b and outlet 46 thereof for pressuring fuel from the pipe 45 to a pipe 48. A by-passyalve 46d isadapted toconnect the inlet and outlet around the rotor 46a whenever inlet pressure is greater than outlet pressure. A relief valve 45c is adapted to relieve fuel from the outlet back to the inlet to maintain outlet pressure at a constant pressure differential basedv on either ambient air pressure or engine intake manifold pressure and selectively determined by an adjustable spring 4.6) urging the relief valve toward'closed position. A diaphragm 46g is arfactor for maintaining the selective pressure differential, it is preferred that one side of this diaphragmhave an effective area exposed to the engine manifold pressure through a tube 46h connected to the manifold 49 of the engine 41 that is balanced by an equal area on the valve 46c exposed to the pump outlet pressure.

' The intake manifold 49 of the engine 41 has an air pressure-controlled variable Venturi throat 50 controlling flow of fuel through .a carburetor i receiving the pipe line 46 and having a discharge line 52 connected to the engine intake manifold at a point closer to the engine than the throat 56. An air supercharger or air pump 53 is provided to force air through the Venturi throat 56. Air from the supercharger 53 admixes with fuel from the fuel line 52 in the manifold 49.

. The engine intake manifold 46 has a tube 54 leading therefrom at the point therein adjacent the point where fuel from the carburetor and air from the supercharger are admixed. This tube 54, as shown in Figure 4, communicates with the interior of the cup member 29 in the casing 21 between the wall 29a of the cup and a shoulder 291) on the cup. This shoulder has a diaphragm disk 55 held thereon in sealing relation by means of a washer 56 and .cap screws 55a extending through the washer and threaded into the shoulder 2%. The diaphragm 55 snugly embraces a shaft 51 extending through the cup member and projecting into the cap 28 and also into the chamber 32. Metal disks 58 overlie the central portion of the diaphragm 55 on both faces thereof and are abutted by tubes 59 seated on the shaft 51. Each of these tubes has outturned flanges at the ends thereof, one set of which engage the washers or disks 58 and .the other set of Which engage small diaphragms 65 respectively carried around their peripheries by the partition wall 29a of the cup 29 and by an apertured metal disk 6! between the end cap 28 and the open end of the cup 29. The shaft 51 has an integral collar portion or head 51a spaced from the end thereof which projects into the chamber 32 effective to clamp the adjacent diaphragm 60 between the end of a tube 59 and the collar 51a thereby providing a sealed compartment 30a in the chamber 36 on one side of the diaphragm 55. The other small diaphragm 66 is sealingly clamped by the other tube member 59 and by a ferrule or tube 62. The ferrule o-r'tube 62 is acted on by a nut 63 threaded on the shaft 51. The diaphragm seal 66 thus provides a second compartment 301) in the chamber 39 on the other side of the diaphragm 55. A spring retainer 64 overlies the seal 60 on the plate 61 and receives a coiled spring 65 therearound. A second spring retainer 66 receives the other endof the coiled spring 65 and a nut 61 on the end of the shaft 51 adjustably positions the retainer 66 to control compression of the spring 65. The spring 65 tends to urge the shaft 51 away from the chamber 32 into which said shaft projects. The end cap 28 defines a recess receiving the spring retainers and spring and the end of the end cap 23 is preferably apertured as at 28a. so that the nut 61 is accessible from the outside to control the compression of the'spring 65.

1 :A tube 68 joins the compartment 3% with the discharge side or'outlet 44 of the pump. i ..:lThus one side of the diaphragm. 55 is exposed to engine intake manifold pressure through the tube 54 tending to shift the shaft 51 out of the chamber 32. The other side of the diaphragm 55 is exposed to pump discharge pressure through the tube 68 tending to shift the shaft 51 further into the chamber 32. The diaphragm seals 66 permit shifting of the shaft while sealing the compartments 30a and 30?) on opposite sides of the diaphragm 55.

The compression of the spring 65 can be set by the position of the nut 61 to oppose, with any desired force, shifting of the shaft 51 further into the chamber 32 and thereby opposing the pump discharge pressure acting through the tube 68 on the diaphragm 65 from shifting the shaft. As a result of this adjustment, once the spring is set to maintain a desired pressure differential between pump discharge pressure and engine intake manifold pressure, the shaft will only be shifted upon variation of the pressure differential between the compartments 36a and 3% which compartments are respectively under manifold pressure and pump pressure.

A disk 16 has a hub portion 1| keyed or otherwise secured to the motor shaft 36 in the chamber 32 of the casing 21. This disk has an upstanding circular flange 12 surrounding the same and providing an open cup facing the partition wall 29a of the cup 29. The other face of the disk carries a pair of segmental circular contact rings 13 as best shown in Figure 6, Two brushes such as 14 carried in ferrules such as 15 by the partition wall 31 are spring-urged against the contact rings 13.

As shown in Figures 4 and 5, the face of the disk 16 opposite the face carrying the contact rings 13 carries a pair of opposed brackets 16 and 11. The bracket 16 carries a leaf spring 18 while the bracket 11 carries a leaf spring 19. The leaf springs 18 and 19 project across the face of the disk in spaced opposed parallel relation with the spring 18 being radially outward of the spring 19. The spring 18 carries a contact point and the spring 19 carries a contact point 8| cooperating with the point 86.

A bracket 82 is secured on the disk 16 and tiltably carries a lever 83 with a button end 84 acted on by the end of the shaft 51. An arm 65 is provided on the lever 83 to actuate a rod or link 86 secured at its end tolthe outer contact point 80. When the shaft 51 is moved further into the chamber 32, it will tilt the lever 83 and arm 85 to actuate the rod 86 for drawing the contact point 80 radially inward.

The position of the contact point 3| is influenced by centrifugal force and depends upon the speed at which the disk 10 is being rotated.

As shown in the wiring diagram of Figure 3, current from a battery or other source 61 is fed through a wire 88 through both the stator 34 and rotor 35 of the electric motor for the emergency booster pump 26. A wire 89 connects the stator or field 34 with the contact rings 13 through one brush 14. The contact rings 13 are joined through a circuit 90 including additional field windings or resistance 9| in series with the field 34 whenever the contact points 80 and 6| are separated. A wire 92 connects the armature 35 and contact rings 13 through the other brush 14 with the ground 93,

. When the contact points 80 and 8| are together, the resistance 91 is removed from the circuit 90. L j

. Since a motor with a shunt field is used, addition of resistance 9| to the field will weaken the field and cause the motor to operate at increased speed. Therefore, whenever the points 80 and 8] are separated, the motor will be operating at high speed. Whenever the points 80 and BI are together, resistance is taken out of the field circuit, the field is strengthened, and the motor will operate at reduced speed.

As explained in connection with Figures 4 and 5, the radial position of the contact point M. is controlled by the speed of the disk T under the influence of centrifugal force since the contact point 81 will tend to fly outwardly. However, the radial position of the contact point 80 is controlled through the link rod 86 and lever 83 by the position of the shaft 51. Thereforethe speed at which the contact point 8| will reach the contact point 80 depends upon the position of the shaft 51.

As explained hereinabove, the position of the shaft '51 is controlled by the spring 65 and by the difference in pressure or pressure differential between the compartments 30a and 36b. The compartment 30a is under engine intake manifold pressure while the compartment 3% is under pump discharge pressure, Therefore adjustment of the spring 65 will predetermine the speed at which the contact points will close and will serve to maintain a predetermined pressure differential between pump discharge pressure and engine intake manifold pressure. If the pump discharge pressure increases above the desired differential, the diaphragm 55 will shift the shaft 51 further into the chamber 32 thereby drawing the contact point 80 radially inward and maintaining closed position of the points at lower speeds. The motor will thereby slow down and the pump pressure will be decreased. On the other hand, if the intake engine manifold pressure should increase relative to the pump discharge pressure, the action of the spring 65 to retract the shaft from the chamber 32 will be assisted by piessure in the compartment 30a. This will permit the contact point 80 to assume a more outward position under the influence of centrifugal force and the contact points will not be closed until a higher speed is reached thereby speeding up the motor and increasing the pump pressure to bring it up to a predetermined differential above the increased engine intake manifold pressure.

Thus, the diaphragm and centrifugal governor control for theemergency pump unit 25 make possible the use of this unit in place of the engine driven main pump 16 in the event that the pump d6 becomes inoperative and in such event the engine will continue to receive suflicient fuel under the desired pressure to maintain high altitude flight the same as when the main pump is operatting. Ihe emergency pump can maintain the same fuel pressure differential as the main pump based either on ambient air pressure or engine manifold pressure.

From the above descriptions it should be-understood that the invention provides a tandem or series arrangement of pumps including an emergency pump between other pumps in the system which emergency pump can be operated to maintain a predetermined pressure differential of the material being pumped. This pressure differential is preferably based on engine intake manifold pressure of an aircraft engine but without departing from the principles of the invention 'it can be based on. ambient air pressure by merely exposing the compartment 30a to the ambient air instead of to the engine intake manifold.

At least two of the pumps in tandem are centrifugal type pumps having inlets and outlets in constant communication so that fiuid can flow therethrough even when the pumps are inoperative. One of the pumps may be a positive dis placement pump with a by-pass therearound.

When the invention is embodied in an aircraft fuel system, a centrifugal type pumpis mounted on each fuel tank of the aircraft and is driven by an individual electric motor. These pumps have agitators associated therewith disposed in the tanks'to eliminate gases and vapors from the liquid about to enter the pumps so that vapor lock will not occur. The tank mounted pumps discharge through a selector valve having a plurality of inlets which can be placed in communication with each other together with an outlet communicating with the emergency pump. The tank-mounted pumps can thus be used to transfer fuel from one tank to another by positioning the selector valve 2|, so that two of its inlets are in communication.

It will, of course, be understood that various details of construction may be varied through a wide range without departing from the principles of this invention and it is, therefore, not the purpose to limit the atent granted hereon otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. In a fuel system adapted for high altitude aircraft performance, an emergency electric motor-booster pump unit having a pump inlet and an outlet in constant communication with the inlet, a main fuel pump having an inlet communicating with the outlet of the emergency booster pump and having a by-pass therearound, an air supercharger, a carburetor receiving fuel from the main pump, an engine intake manifold receiving air and fuel from the supercharger and carburetor, said electric motor of the emergency booster pump unit having a shunt field, a centrifugal governor driven by said shunt field motor having contact points in the emergency electric motor circuit that are mounted for swinging .radially outward under the influence of. centrifugal force, a diaphragm exposed on one side to discharge pressure of said emergency booster pump and exposed on the other side to pressurein the intake engine manifold at the point where the fuel and air are mixed, means controlled by said diaphragm for regulating the radial position of one of said contact points, and a resistance circuit adapted to be placed in series with the shunt field of the electric motor driving th emergency booster pump whenever said contact points are separated whereby the field is weakened and the motor speed is increased.

2. In a fuel system including a positive dis placement fuel pump. a carburetor receiving fuel from said pump, a by-pass associated with said pump, and a device operated in one direction by air pressure and in the reverse directionby fuel pressure associated with said pump tomaintain a constant pressure differential between the fuel pressure at the carburetor inlet and the air pressure existing at the pointwhere the fuel is introduced into the air for forming a combustible mixture; the improvement of a centrifugal pump having an outlet communicating with the inlet ofthe fuel pump. a driver for said centrifugal pump, and automatic means for controlling. the'speedof saidfdriverexposed c'ri one "side toilcentrifugal pump'pressure' and on' theother side to pressure existing at the pointwhere fuel is introduced into the air' for forming a combustible mixture w'h'ereby said centrifugal pump can take the place of said fuel pump and device to maintain the constant fuel pressure differential.

3. A pump and motor unit comprising anelectrio-motor, a pump impeller driven by said electric motor, means defining a pumping chamber surrounding said impeller" and having a central inlet and a peripheral' outlet acentrifugal gov ernor driven by said electric fmotor including swingable contacts adapted to yieldunder centrifugalinfiuenceto controlthe speedjof the motor, and mean for" automatically adjusting the contacts of said governor in accordance with the pum discharge pressure to control the speed of the motor for maintaining a constant pump discharge pressure. p

'4. A pump and electric motor unit comprising an electric motor, a pumpfimpeller driven by said motor, means providing a pumping chamber surrounding said impellerga resistance circuit, a centrifugal governor including swingable contacts in said circuit adapted to yield under centrifugal influence for placing said resistance circuit in and out of series with said electric motor to control the speed of the motor, and means automatically adjusting the contacts of said governor in accordance with the pressure differential between the pump discharge pressure and a base pressure for regulating the speed of the motor to maintain a constant pump pressure differential.

5. A unit comprising an electric motor having an armature and a shunt field, a pump driven by the armature of said motor, a centrifugal governor driven by the armature of said motor, a contact point swingably mounted on said governor and adapted to swing outwardly under the influence of centrifugal force, a cooperating contact point swingably mounted on said governor, contact rings on said governor, brushes cooperating with said contact rings connecting the same with the field and armature of said motor, a resistance circuit interposed between said contact rings to add resistance to the field of said motor, said contact points arranged to open and close said resistance circuit, and diaphragm means exr posed to pump discharge pressure on one side and a base pressure on the other side controlling the radial position of one of said contact points.

6. A pump and motor unit comprising a casing defining a diaphragm chamber, a governor chamher, a motor chamber, and a pumping chamber having an inlet and an outlet, a pump impeller in said pumping chamber between said inlet and outlet, an electric motor in said motor chamber driving said pump impeller, a centrifugal governor in said governor chamber driven by said electric motor, a diaphragm in said diaphragm chamber dividing the chamber into opposed compartments. means controlled by said diaphragm acting on said governor to control the governor in accordanc with the pressure diiferential between the two compartments, and means connecting one of said compartments with the pump outlet.

'7. A pump and motor unit comprising an electric motor, a drive shaft driven by said motor, a pump impeller driven by said drive shaft, a disk driven by said drive shaft, a leaf spring carried by said disk, a contact point on said leaf spring adapted to swing outwardly under the influence of centrifugal force, a second contact pointbn saiddisk, a leverc'ontrolling the radial position of said second contact point, a dia-' phragm actuating said lever exposed on one side to pumpdischarge pressure, and means controlled by said contact pointsto regulate the speed of said; motor. l

' 8: In a pump and electric motor unit a pump casing defining a volute chamber with a central inlet and a: peripheral outlet, an impeller in, saidvolute chamber for pressuring fluids from the inlet to the outletyan electric motor having an armature driving'said impeller, and a shunt field-cooperatingwith saidarmature, a resist ance circuit'for strengthening and weakening said field, swingable contact 'devicesfthat are centrifugally influenced for controlling said en: cult, and a devi'ce exposed to pump discharge pressure on: one side and air pressure 'on the other side for controlling said contact points.

9.In"atpu'mp unit an electric motor having a fieldya resistancecircuit in "said field, a pump driven by said'motor, a governor driven by said motor including "a swingably" mounted contact point adapted to move outwardly under" the influence of centrifugal force and acoopratirig swingably mounted contact point, a spring controlling the position of said cooperating contact point, and a diaphragm coacting with said spring exposed to pump discharge pressure on one side and base pressure on the other side, said contact points when opened adding said resistance circuit to the field of the electric motor causing the motor speed to increase and said contact points when together removing said resistance circuit from the field of the electric motor to slow down the speed of the motor whereby the pump is driven at speeds to maintain a pressure differential above base pressure predetermined by said spring. 7

10 In a pump system, a centrifugal pump and a main pump in tandem, a device associated with said main pump for maintaining a selected pressure differential above base pressure at the outlet of the main pump, a driver for said centrifugal pump, and an automatic speed control for said driver causing the driver to operate the centrifugal pump at speeds producing the same pressure differential as the main pump.

11. In a pump system, a plurality of pumps in tandem, a device associated with one of said pumps for maintaining a selected pressure dif-.

ferential above base pressure at the outlet thereof, a driver for another of said pumps, and an automatic speed control for said driver for causing the driver to operate said another pump at speeds producing the same pressur difierential as said one pump.

12. In a fuel system, an engine driven fuel pump having a device associated therewith for maintaining a desired differential above a selected base pressure at the outlet thereof and a by-pass therearound, a booster pump, conduit means connecting the outlet of the booster pump with th inlet of the fuel pump, a driver for said booster pump, and means actuated by variations in said pressure differential to control the speed of said driver for maintaining the desired pressure differential at the outlet of the fuel pump.

13. In a fuel system adapted for an aircraft engine including an engine-driven positive displacement main pump having a by pass means therearound, a relief valve associated with said pump, a diaphragm movable with said relief valve having one face exposed to pump inlet 11 pressure and the otherface exposed to; a selected base pressure, a spring urging said. valve; toward closed position. to maintain a selected pressure differential above the; selected base pressure at the outlet of the pump, a fuel tank;. and-a booster pump receiving fuel from said tank for pressuring; the fuel to .the intake of the main fuel pump; theimprovement. of an emergency centrifugal type pump having: an inlet receiving fuel from the. booster pump and an outlet for supplying fuelto the, intakeqof the main fuel pump, an electric motor driving Esaid centrifugal pump, and automatic means including. a. diaphragm having one: face exposed to the" selected. base pressure and the-other face exposed to the pressure at. the

' outlet of the; centrifugal. pump for controlling the-speed of the electricv motor to cause. the cen-- trifugal pump to pressure fuel at the sam pres.- sure differential. produced by the main, pump.

; 14-. In a fuel. system for an. aircraft. engine ineluding: a fuel tank, a boosterpump having: an inlet receiving fuel from said: tank and an outlet in constant communication with said inlet, a; main fuel: pump having an. inlet receiving. fuel from. the outlet; of said booster pump and: an

1'2 outlet. communicating with the aircraft engine, a. by-pass around saidmain fuel pump. connecting said'inlet and said outlet thereof, a relief valve'in said main fuel pump, a diaphragm movable with said valve having one. face exposed to inlet pressure and the other face exposed to a selected base pressure, a spring urging said relief. valve to closed;position for maintaining a selected pressure differential at the outlet of said pumpv above the selected base pressure, an electric motor driven centrifugal booster pump hav: ing an inlet communicating. with the outletv of the. first mentioned booster pumpand an outlet communicating with the inlet of..the main. fuel pump, and a centrifugally actuated electric means associated with saidelectricc motor for controlling thespeed of said. motor, said means including a diaphragm having one fac vented to the same selected base pressure as the diaphragm of the main fuel pump and the other face vented to outlet pressure of the centrifugal booster pump whereby said electric motor will be driven at a speed for maintaining a constant pressure differential at the outlet of the main fuel pump.

RUSSELL R. CURTIS; 

